There are many applications for combining lasers and other solid-state light sources. In general, when an application requires more power than can be delivered by a single laser source, a common solution is to combine the light from two or more lasers of the same wavelength. Since the additional lasers are in a physically different location, it becomes necessary to combine and stack the laser output beams together, eliminating as much “dead space” as possible in the combined output beam.
When lasers are combined in this manner, it is often desirable to make the combined source as small as possible (i.e., to have the smallest possible etendue) so that the energy of the combined beam can be effectively and efficiently concentrated and transferred to another optical system. Combining methods use various characteristics of the light including wavelength, polarization, and spatial characteristics.
One application for which spatial combining of multiple sources is of particular interest is in pump excitation for fiber lasers. In a fiber laser, the active gain medium is an optical fiber doped with suitable rare-earth elements. Pump energy can be provided from a number of types of sources, such as using a set of multiple laser diodes that are fiber-coupled to the gain medium. By using multiple pump sources, higher optical power can be directed to the gain medium. The use of multiple laser sources also allows each of the pump lasers to operate at a lower power level for a given amplifier gain, thereby extending the lifetime of the pump lasers and hence the reliability of the amplifier. This also provides some redundancy in the event that one of the pump lasers fails.
Because a single wavelength is needed for pump energy, the individual sources must be closely matched, making laser diodes a practical choice. However, laser diodes do not provide a beam that is circular in cross section, that is, with highly symmetrical energy distribution about a central axis. Instead, the aspect ratio of the output light is highly asymmetric, with markedly different divergence angles in orthogonal directions, generating an output beam whose length (considered to be along the “slow” axis) can be several times its width (along the “fast” axis). This asymmetric characteristic makes it desirable to stack the component output beams as closely together as possible, to form a composite beam with a more nearly symmetric aspect ratio. As a limiting factor, the input optical fiber for accepting the pump energy has a relatively small numerical aperture (N.A.), which limits the angular extent of the incoming composite beam and makes it desirable to eliminate as much dead space between component beams as possible.
Among solutions that have been implemented or proposed for combining laser sources for use as pump lasers is a modular pump module with vertically staggered laser diodes and corresponding mirrors. FIGS. 1A and 1B show top and side views, respectively, of a typical pump module 10 of this type. In this approach, each of three lasers 12a, 12b, and 12c directs a beam through a corresponding set of crossed cylindrical lenses 15a, 15b, and 15c for a first axis (the fast axis FA, as described in more detail subsequently) and 14a, 14b, and 14c for the orthogonal axis (the slow axis SA, described subsequently) and to a mirror 16a, 16b, and 16c, respectively. A filter 30 provides a measure of protection from feedback light FB, as described in more detail subsequently. A composite beam 28 that is inside a pupil 27 of a lens 18 is then focused by lens 18 into an optical fiber 20 for use as pump energy.
As the side view of FIG. 1B shows, with vertical distance intentionally exaggerated for clarity, the lasers 12a, 12b, and 12c and their corresponding cylindrical lenses 14a, 14b, and 14c and three spaced-apart mirrors 16a, 16b, and 16c are vertically staggered, by a slight distance. This arrangement of reflective components leaves little tolerance room between the component output beams. The light from laser 12a is clipped by the top of mirror 16b, for example. Similarly, the light from laser 12b is clipped by the top of mirror 16c. The inset W in FIG. 1A shows the angular distribution of composite beam 28 at the pupil of lens 18. Inset W shows how composite beam 28 is formed, with an output beam 22a′ from laser 12a, an output beam 22b′ from laser 12b, and an output beam 22c′ from laser 12c. There is necessarily some gap distance or dead space 24 between the stacked output beams due to tolerances needed to pass the beams by the fold mirrors 16b and 16c. 
While the solution described with reference to FIGS. 1A and 1B has proved to be workable, there is room for improvement. Manufacturing tolerances are tight, with little room for variability in fabrication. Each component must be precisely aligned, so that the light is properly redirected from mirrors 16a, 16b, and 16c. Because each laser reflects off a different mirror, thermal variations between the mirrors 16a, 16b, and 16c adversely affect the alignment of the system during operation. Significantly, for practical reasons, this type of solution allows only a restricted number of lasers, three or fewer, to be combined.
With conventional combining solutions, the aspect ratio of the composite output beam is poorly matched to the design of the combining system. In FIG. 1A, an inset Z shows an overlapped image 23 of each laser mode field at an input aperture 90 of optical fiber 20. Image 23 is formed from the superimposed or overlapping images of the spatial distribution of the output of the lasers, shown as images 122a′, 122b′, and 122c′. A number of observations can be made:                (i) Input aperture 90 is substantially filled in the slow axis SA direction of each beam. A longer beam in the SA direction would simply overfill the input aperture of the fiber in that direction.        (ii) Input aperture 90 is under-filled in the fast axis direction FA.        (iii) Inset W shows that the pupil of lens 18 in the SA direction is also filled.        
The first observation (i) represents a constraint on the manufacture of lasers used for generating pump energy. Lasers that provide an even longer slow axis than those currently available could be readily manufactured and would provide higher power and efficiency; however, there is no way to take advantage of this potential capability with conventional pump module combiner designs because there is no additional capacity for light; in that axis, the fiber etendue is constrained. The second observation (ii) above shows that there is additional room in the etendue of the fiber to add more light, but this space is in the fast axis direction of the laser, which is much brighter than the slow axis. Further, there is little tolerance for alignment in the slow axis direction, since the light beam already tends to overfill the aperture in this direction.
Solutions that have been proposed for improving efficiency of the laser pump module include superimposing additional beams into the fiber, such as by changing polarization for a set of beams and overlapping the beam paths of different polarizations in order to use light from additional lasers. Such methods, however, face the same etendue-based limitations described with reference to FIG. 1A. The size of the usable laser mode field is constrained. Because the aspect ratio of the etendue of light from the laser diodes is unchanged, these solutions still place constraints on output beam dimensions. Thus, even though a longer laser diode than those currently used for laser pumping could be fabricated and would be more efficient with respect to light output and less costly, conventional light multiplexing solutions are unable to take advantage of the potential efficiency gains and reduced cost such designs could provide. One result of this etendue-based constraint is to make it impractical to provide laser diode designs having improved efficiency.
Thus, it can be seen that there is a need for a method and apparatus for spatially combining light sources that provides an improved match between the etendue characteristics of a single laser light source or of combined laser light sources and the etendue of an optical fiber, such as an optical fiber used to provide pump light energy to a laser.